The polypeptide pattern of intracytoplasmic A particles, isolated from mammary tumors of ICRC mice, displayed a prominant doublet of about 75,000 molecular weight (Ap75) and a lesser band of 42,000 molecular weight (Ap42). A particle proteins were analyzed by tryptic fingerprint mapping and compared to similarly treated core proteins (p10, p14, p23, and p28) of murine mammary tumor virus (MuMTV) purified from the milk of ICRC mice. Comparison of the peptide maps showed that almost all the major spots of the MuMTV core proteins could be located within the map of Ap75, and that Ap42 contained peptides of only p28 and p14. Since tryptic peptide maps of MuMTV core proteins have been shown to be strain-specific markers, we conclude that the A particles present in mammary tumors of ICRC mice are coded for by the same genes that code for the virions present in the milk of these mice.
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